Various types of semiconductor devices are manufactured in much the same way. A starting substrate, usually a thin wafer of silicon or gallium arsenide, is masked, etched, and doped through several process steps, the steps depending on the type of devices being manufactured. This process yields a number of die on each wafer produced. The die are separated with a die saw, and then packaged into individual components.
During the plastic packaging process, several semiconductor die are attached to a lead frame, often with a material such as metal, an epoxy, a thermoset, a thermoplastic, or other viscous adhesives. Bond wires couple each of several terminals (bond pads) on each die to conductive leads on the lead frame. The die, the wires, and a portion of the leads are encapsulated in plastic. These leads couple the die with the device into which the component is installed, thereby forming a means of input/output (I/O) between the die and the device.
Bond pads historically have been located around the periphery of the semiconductor die. This allows the lead fingers of the lead frame to be placed very near the edge of the die, and therefore very short bond wires are required to attach the bond pads to the lead fingers. This configuration requires a lead frame paddle, however, to which is attached the semiconductor die, and the use of a die paddle is known to have several problems For example, the metal of the paddle and the material from which the die is manufactured have different coefficients of expansion. If an adhesive is not carefully chosen, the die can be damaged during temperature cycling from stress placed upon it by the expanding and contracting paddle. Also, the lead fingers are typically plated with gold or silver to facilitate bonding with the bond wires, and the paddle is also unnecessarily plated, thereby increasing the cost of production.
One way to solve these as well as other problems associated with the lead frame paddle is described in U.S. Pat. No. 4,862,245 by Pashby, et al. which is incorporated herein by reference. Pashby, et al. describe a die which allows for a "leads over die" configuration, in which the lead fingers of the lead frame are attached to the top of the die, then the bond pads are coupled with the lead fingers by bond wires. A leads over die configuration has the advantage of not requiring a die paddle. It also allows for a smaller package, as well as other advantages described therein.
One step of semiconductor manufacture using the die of Pashby that is not without problems is the die-lead frame attachment. Die-lead frame attachment, as suggested by Pashby, includes an alpha barrier of polymeric film coated on both sides with an adhesive material selected from the group of epoxies, acrylics, silicones and polyimides For the die-alpha barrier attachment, the silicone adhesive is recommended since the silicones minimize corrosion. For the die-lead frame attachment, an epoxy Or acrylic is recommended since these materials assure that the conductors are fully bonded to the alpha barrier, thereby enhancing the thermal conductivity between the semiconductor chip and the conductors, and mechanically locking the lead frame conductors to the semiconductor chip.
A problem which can occur by using any of the referenced materials results from their hard-cure properties. The epoxies and acrylics both harden immediately upon curing to form a somewhat "brittle" attachment. The silicones and polyimides also tend to cure with the repeated application of heat such as that typically found during the formation of a semiconductor device, for example at wire bond, encapsulation, and wave solder, although their attachment is somewhat more pliable than that of the acrylics and epoxies Using any of the materials, once any of the adhesive materials set up it is no longer possible to return them to their precured viscous state.
Once the materials set up, a stress can result between the die and the adhesive material and between the lead frame and the adhesive material resulting from temperature variations and differences in the thermal expansion coefficients between the adhesives and the materials to which they are attached. For example, the metal material of the lead frame expands and contracts at a greater rate than the acrylics and epoxies, and the lead fingers extending over the surface of the die can separate from the adhesive material due to these stresses. Contributing to the separation of the lead fingers from the adhesive is the small size of the lead fingers which results in a small contact area with the adhesive. For this reason, the adhesive which connects the lead fingers to the alpha barrier must be stronger than the adhesive which connects the die to the alpha barrier, as the die to alpha barrier connection has a greater surface area. Damage of the circuitry on the surface of the die can result as the metal circuitry expands and contracts at a greater rate than the adhesive material
Thermoset materials have also been used to connect the die to the lead frame. As is known in the art the thermoset materials are viscous at low temperatures and cure with the application of heat. Once the materials cure they are no longer capable of softening. The materials can cure at various temperatures, depending on the material. In any case, with the application of additional heat, the materials further harden. This material can create problems similar to the acrylics and epoxies in that differences in thermal expansion coefficients with the die and lead frame can cause stress on the surface of the die.
Thermoplastics have also been used to couple the die and the lead frame. As is known in the art, these materials are hard at low temperatures, but soften and flow at elevated temperatures. As they cool, they again harden, and again soften with the application of heat. These materials can solve the stress problems associated with the materials described above, as they soften with the application of heat, and therefore the stress is relieved as the die circuitry and lead frame materials expand and the material softens. An identical thermoplastic is conventionally used on both sides of the polyimide carrier material. A problem that can occur when using a thermoplastic to couple the die and the lead frame results from the die contact area being greater than the contact area of the lead frame to the thermoplastic. A thermoplastic which softens at a temperature optimized for the die-alpha barrier attachment can result in separation of the lead frame from the alpha barrier as the softening point is reached, for example during wire bond or at elevated temperatures. Alternately, a thermoplastic optimized for the lead frame-alpha barrier attachment can result in stress on the surface of the die.
An assembly which has reduced susceptibility to separation of the lead frame from the alpha barrier and reduced stress to the surface of the die would be a desirable structure.